In a typical cellular radio system, user wireless terminals communicate via a radio access network (RAN) to one or more core networks. The user terminals can be mobile stations such as mobile/cellular telephones, laptops/notebooks with mobile capabilities, and other portable, pocket, hand-held, or car-mounted mobile devices, which communicate voice, data and/or video with a radio access network. Alternatively, the wireless units can be fixed devices, e.g. fixed cellular terminals which are part of a wireless local loop or the like.
Clock synchronization is very critical for digital communication networks; the clock (also knows as the local oscillator) at the receiver end of a communication link must be well synchronized with the clock at the transmitter end both in time and frequency so that it can extract the signal at the right time and at the right frequency to be able to then reconstruct the signal properly.
Furthermore, when multiple user terminals communicate concomitantly with a base transmission station (BTS), scheduling of the transmission and reception time slots requires that all nodes (BTS, user terminals) of the network be synchronized in both time and frequency. Conventionally, wireless standards specify guard intervals (time, frequency) to regulate the tolerable mismatches in time or/and frequency between the two ends of a communication link. Without proper synchronization, the mismatch degrades the system performance and therefore results in an unsatisfactory quality of service. A better synchronization reduces the amount of drifting of bursts of packets beyond a defined transmission period and limits channel frequency drifts, which results in enhanced quality of the received signal and therefore in a better decoding performance.
In wired (or wire-line) networks, the global clock is usually provided using a network timing reference (NTR), and the terminals, or the nodes, needs to synchronize their own clock to the NTR. In wireless cellular communication, the global clock is usually provided to the user terminals (user equipment) units by a serving base station (BTS) via in-band signaling; a BTS transmits regularly or continuously a beacon or pilot signal based on an internal clock. The internal clock can be locally generated, derived from an infrastructure network (from legacy T1 or E1 carriers), or synthesized from an external clock. User terminals/equipment will always search for a network clock and then synchronize their individual clock with that clock and constantly track it. Wireless networks may be asynchronous or synchronous. For example, GSM (Groupe Special Mobile) systems are asynchronous and therefore the GSM terminals retrieve different clocks from different BTS. CDMA (Code Division Multiple Access) networks are synchronized in that they use a GPS (Global Positioning System) clock, so that the CDMA terminals retrieve the same clock from any CDMA BTS. Other synchronous networks are not synchronized onto GPS time; they rather use a master clock.
Equipment manufacturers and network providers are currently examining several options for achieving better synchronization. A number of main techniques currently used or under consideration: Adaptive Clock Recovery (ACR), Synchronous Ethernet, Network Time Protocol (NTP) and Precision Time Protocol (PTP). The ACR algorithms attempt to reproduce the master network clock at far-away nodes. Though ACR-based techniques are seeing some interest, the proprietary aspect of this solution makes manufacturers and providers skeptical of using them. The ITU (International Telecommunications Union) is defining a standard for Synchronous Ethernet as a way to synchronize frequencies over Ethernet networks. But Synchronous Ethernet will be suitable only for new applications because all elements in the network will need to be significantly upgraded to support the standard. NTP is the most widely used protocol for time synchronization over LANs and WANs. It is one of the oldest Internet protocols still in use. NTP is relatively inexpensive to implement, requiring little in the way of hardware. It can usually maintain time synchronization within 10 milliseconds over the public Internet and can attain accuracies of 200 sec or better in LANs under ideal conditions. However, the current version of NTP does not meet the higher precision requirements for Internet evolution, particularly for wireless Internet with latency critical applications.
IEEE 1588 Standard (Precision Clock Synchronization Protocol for Networked Measurement and Control Systems), also known as PTP (Precision Time Protocol) has received considerable attention since its introduction in 2002. It forms the basis for defining Ethernet links that can transport synchronization signals with small and well-defined delays (with accuracy on the order of sub-milliseconds), synchronizing Ethernet tasks over large physical distances. A variety of silicon vendors are now producing hardware that supports PTP. PTP is used in telecom are for example in LANs supporting multicast communications over heterogeneous systems that require clocks with varying resolution and stability.
The PTP clocks are organized in a master-slave hierarchy, where each slave synchronizes to its master based on a small set of messages exchanged between the master and slave. Thus, the master sends to the slave synchronization messages that include the sending time and measures the time difference between the master and slave clocks using the response messages received from the slave. Similarly, the slave sends to the master delay request messages that contain the estimate of the sending time and measures the time difference between the slave and master clocks. The one-way delay between the clocks and the offset of the slave clock can be then determined based on two measurements, enabling the slave to correct its clock based on the offset. All clocks run a best master clock algorithm.
PTP can coexist with normal network traffic on standard Ethernet using transparent switches and 1588 boundary clocks. A boundary clock simply serves as a time-transfer standard between the subnets defined by routers or other network devices. The boundary clock has a network connection to each of the subnets. Ordinary clocks within each subnet synchronize with the boundary clock. The boundary clock resolves all of the times of the several subnets by establishing a parent-child hierarchy of clocks. However, use of cascading boundary clocks can cause nonlinear time offsets to accumulate in the servo loops that generate these clock signals, degrading their accuracy to an unacceptable degree.
Another current trend is to equip all BTSs with a GPS clock, including the BTSs serving non-CDMA networks.
New wireless technologies such as 3G (third generation) or fourth generation and B3G (beyond 3G) are being developed with a view to enable network operators to offer users a wider range of more advanced services while achieving greater network capacity through improved spectral efficiency. Also, one of the most significant features of 3G mobile technology is that it supports greater numbers of voice and data customers, especially in urban areas, and higher data rates at lower incremental cost than 2G. Services they can offer include wide-area wireless voice telephony and broadband wireless data.
It is another current trend for operators to investigate the possibility of providing a small wireless network within a home, or a small area of coverage with a limited number of users. Such a small network includes a small radio base station (RBS), also called a “femto RBS”, (the term “femto” intends to indicate that the coverage area is relatively small), or “home RBS” that provides coverage over a “femto cell” for the end users when at home or inside a building where the wireless signals are significantly weaker than that outside. There are different architectures proposed for such femto-cells.
To summarize, different communication networks have different synchronization specifications and different services require different synchronization accuracies. A synchronization process usually needs a few hundreds milliseconds to plural seconds even minutes, and also the nodes need to be equipped with a tracking mechanism. This diversity results in implementation difficulties in wireless devices and intermediate equipment and results in wasteful use of system resources due to in-band signaling and complexity of the actual implementations of the synchronization functionality. The emerging technologies and systems must take these issues into account and provide for better use of available resources and enable better services at lower costs.
Therefore, there is a need to improve synchronization within an wireless access network and among wireless communications networks in general, both with a view to enhance the services offered to mobile device users and to provide for a better use of the available resources (such as bandwidth). This need is more relevant to emerging femto-cell technologies and to the respective home electronic devices.